RFR: JDK-8306990: The guarantees given by Region's floor and ceiling functions should work for larger values

[Kevin Rushforth]

On Thu, 27 Apr 2023 10:11:42 GMT, John Hendrikx <jhendrikx at openjdk.org> wrote:

> Region has floor and ceiling functions that ensure that calling them twice in a row will yield the same result:
> 
>      ceil(x) = ceil(ceil(x))
> 
> However, due to use of a constant `EPSILON` which is added/subtracted before doing the rounding, this only works for small numbers (in the range of 0-50 approximately).  For larger values and scales, rounding errors can easily occur.  This is visible as artifacts on screen where controls are a pixel wider than they should be.
> 
> The use of the `EPSILON` constant is incorrect, as its value depends on the magnitude of the value in question (as magnitude increases, the fractional precision decreases).
> 
> The Math class offers the function `ulp` that should be used here.  It represents the smallest possible change in value for a given double.
> 
> Extending the existing test case `snappingASnappedValueGivesTheSameValueTest` to use larger magnitude numbers exposes the problems.

At first glance, this fix looks correct to me. We'll want to do a reasonable amount of testing.

@andy-goryachev-oracle can you also review this?

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PR Comment: https://git.openjdk.org/jfx/pull/1118#issuecomment-1525543877
PR Comment: https://git.openjdk.org/jfx/pull/1118#issuecomment-1525544287